In general, it is a design challenge to achieve high continuous current ratings for medium and high voltage switchgear at a reasonable cost. Continuous current rating is governed by the heat rise within the switchgear conductors. High current flow in switchgear conductors generates heat. At specific points in the current path, e.g., the primary bushing contacts, hot spots occur due to localized high resistance.
It is generally desired to limit the maximum temperature of the hottest spot on the switchgear bus system to 105° C. (a rise of 65° C. over an assumed ambient temperature of 40° C.), as directed by the standard IEEE 37.20.2. Typical medium and high-voltage metal-clad switchgear arrangements have maximum continuous current ratings of about 3000 A, due to heat generation. It is desirable to increase this current rating. As may be appreciated, it may also be desirable to manipulate different switchgear parameters, such as conductor cross-section or switchgear size, using cooling without concern for increasing maximum current rating.
It is possible to deal with hot spots and thereby reach higher continuous current ratings by a number of means—for example by increasing cross section of the current carrying bus, by increasing the size of the switchgear compartments, etc. One common solution is to use forced air cooling with fans mounted in every third or fourth switchgear frame. The former solutions generally result in unacceptable cost and/or size of the switchgear enclosure. The latter solution is not practical in many cases, including in the case of arc-resistant switchgear, since the byproducts of an arc fault must be contained within the switchgear.
Thus, there is a need to provide an effective passive cooling arrangement for a switchgear.